This invention relates to mass transfer devices in which different fluids are exposed to opposite sides of a semi-permeable membrane so that one or more components of one fluid will pass through the membrane to the other fluid, and more particularly to unique and highly effective material for supporting the flexible membrane material in blood dialyzers for use as artificial kidneys.
Prior to the present invention, a great variety of suggestions have been published for supporting the usually thin and flexible membrane in mass transfer devices, and particularly in dialyzer devices, some of which have been exploited commercially and some of which have not. The present invention will be described primarily in terms of a dialyzer coil for use as an artificial kidney, although it is to be understood that the invention is not limited to the same because the support material of this invention is useful in flat plate and other types of blood dialyzers as well as other types of mass transfer devices.
Dialyzer coil designs which have been exploited commercially utilize mesh or netting material made up of intersecting strands as a support for the flattened tubular membranes. Early coils such as those disclosed in Metz U.S. Pat. No. 2,880,501 issued Apr. 7, 1959, and Broman U.S. Pat. No. 2,969,150 issued Jan. 24, 1961, utilize an over-and-under woven type of mesh material. A significant improvement in commercial dialyzer coil design was brought about by the use of a particular non-woven plastic netting disclosed by Dr. Theodor Kolobow in "A New Dynamic Disposable Artificial Kidney," Transactions, American Society For Artificial Internal Organs, Volume X, pages 116-120 (1964), and Proceedings, Conference on Hemodialysis, (Nov. 9-10, 1964) National Institute of Health, Bethesda, Md., pages 87-94. The same non-woven plastic netting is disclosed as being useful in a multiple-start-spiral design of dialyzer coil in Hoeltzenbein U.S. Pat. No. Re. 27,510 issued Oct. 24, 1972, an improved orientation for the non-woven strands of such netting is disclosed in Miller U.S. Pat. No. 3,508,662 granted Apr. 28, 1970, and a particular cross-sectional shape for such strands is disclosed in Martinez U.S. Pat. No. 3,709,367 issued Jan. 9, 1973.
Membrane support material which does not make use of intersecting strands arranged in the form of a mesh or netting has also been proposed, and used to some extent commercially, although it is believed that such material has not met with any significant commercial success to date in disposable type dialyzers, particularly the popular coil types. Gobel and Bluemle U.S. Pat. No. 3,077,268 granted Feb. 12, 1963, discloses a dialyzer coil utilizing a support material comprising radially impervious plastic sheet having "hobnail" embossments protruding from the opposite sides thereof. To avoid interdigitation of the peaks of the hobnail protrusions which would substantially occlude the blood passage, Gobel and Bluemle provide a series of large protrusions and a thickened separator rim having a longitudinal groove on one side and cooperating disc-shaped projections on the other side to prevent axial shifting of adjacent turns or wraps of the coil. Bluemle U.S. Pat. No. 3,362,540 issued Jan. 9, 1968, discloses a membrane support for a flat plate type of dialyzer which also has embossments protruding from each side of a central sheet, but in this device interdigitation of the opposed protrusions which would substantially occlude the blood passage is avoided as a problem during operation by the use of spacers at the edges of the device. In the coil and flat plate devices disclosed in these patents, the embossments always are arranged in identical geometrical patterns so that the embossments on one side of the blood passage are spaced apart exactly the same along any given directional line as the directly opposed embossments on the other side of the blood passage, and this creates the unacceptable possibility that the opposed embossments can interdigitate and substantially occlude the blood passage, and also the passage for dialysate, unless the special spacing features are provided.
The foregoing problems were solved by the invention described and claimed in Miller U.S. Pat. No. 3,960,730 granted June 1, 1976, by the provision of a membrane support, which does not make use of intersecting strands arranged in the form of either a woven mesh or a non-woven netting, which consists of a sheet of embossed plastic including a large number of protruding embossments on each side arranged in geometrical patterns which are different on opposite sides of the blood passage so that the summits of the embossments on one side of the blood passage are spaced apart differently along any given directional line than the directly opposed summits of the embossments on the other side, so as positively to avoid interdigitation of the summits to an extent which would substantially and undesirably occlude the blood passage.
The embossed membrane support material disclosed in the aforesaid Miller U.S. Pat. No. 3,960,730 has embossments which protrude from each side of the material, as noted, making it possible to angularly offset or otherwise displace or orient the embossed pattern on one side of the sheet with respect to the embossed pattern on the other side. This "double-sided" type of material has disadvantages, however, when compared with substantially uniform thickness material in which each hollow protrusion on one side of the material represents a valley recess on the other side of the material.
Single or uniform thickness material, for instance, has many production advantages over double-sided material. The weight per unit size of uniform thickness material is lower than double-sided embossed material, so that a greater length of single thickness material is included in, say, a 50 pound roll than is the case with double-sided embossed material, which significantly reduces shipping and handling costs. By the same token, the actual amount of plastic membrane support material used in each artificial kidney dialyzer device is less by a small but measurable percentage, similarly reducing cost and weight.
The cost of manufacturing uniform thickness material similarly has advantages over the cost of manufacturing double-sided material. For instance, the manufacture of single thickness material starts with a significantly thinner, oriented sheet of plastic, and the forming of the sheet can be accomplished at a considerably faster rate because there is less deformation required. The longer cooling times which are necessary with the thicker embossments of the double-sided material can be avoided, and there is an absence of thick sections immediately adjacent thin sections which impose constraints on the speed of forming double-sided material.
These and other advantages of using uniform thickness material dictate its desirability for use as a membrane support, but the heretofore unresolved problem is that uniform thickness material, by the very nature of its design, requires that the patterns in which the embossments are arranged must be virtually identical on both sides. This inherent and unavoidable limitation precludes the possibility of having the embossment pattern on one side angularly offset or otherwise displaced with respect to the embossment pattern on the other side of the sheet as taught in the aforesaid Miller U.S. Pat. No. 3,960,730.
Uniform thickness material nevertheless has been proposed for use as a separator in mass transfer devices. Such material is shown in Esmond U.S. Pat. No. 3,738,813 issued June 12, 1973, which discloses a mass transfer device in which the plain gas transfer film 13 is rolled up as a single layer with the embossed support material 15, so that oxygen flows in the space provided between the outer face of the film 13 and the inner face of the layer 15 between its embossments, and blood flows in the space provided between the inner face of the film 13 and the outer face of the layer 15 between its embossments. Since the embossments as formed by the opposed rollers 23, 26 protrude alternately from opposite faces of the sheet 15, however, they of course are in regular patterns such that interdigitation in both axial and circumferential planes is virtually inevitable. Esmond does not suggest that the embossments can be constructed and arranged in predetermined patterns to avoid interdigitation.
Esmond U.S. Pat. No. 3,490,523 issued Jan. 20, 1970, discloses a transfer device which includes a centrally disposed envelope having secured to opposite sides thereof a pair of flow sheets, each flow sheet having a plurality of longitudinal channels of sine-cosine configuration with the opposed curves 180.degree. out of phase defining channels which cross one another at regular intervals and prevent relative collapse. Although suggesting the use of wave patterns to avoid interdigitation, this Esmond patent neither discloses nor suggests that wave patterns can be so dimensioned and arranged as to preclude interdigitation when an elongated length of uniform thickness membrane support sheet is formed into a roll as required in a coil type dialyzer.